10 EMBRYOLOGY OF THE STARFISH. Part I. 



the adult larvaa, and winch is carried downward in its natural attitude while moving;. 

 The dorsal portion of the larva projects beyond the so-called mouth, so that the per- 

 forated extremity has become bevelled • the narrowing of the central portion of the 

 larva has increased, and the digestive cavity which, in younger embryos, occupies 

 the centre of the cylinder (PL I. Figs. 27, 28), is bent towards the loAver side (PI. II. 

 Figs. 2, 4, 5, d). The outer wall has become thickened at a point opposite 

 the bent extremity of the digestive cavity, and the thickening of the wall, together ' 

 with the bending of the digestive cavity, goes on till the closed end touches the 

 lower side at m. 



The changes, which have taken place during the time elapsed since the twentieth 

 hour, have been very gradual. The embryo now enters into a state where the 

 changes are exceedingly rapid and important ; so much so that, at the end of the 

 third day, the embryo has, in a rudimentary state, all the parts which older, fully- 

 developed larvae have. 



At the end of the second day, the reservoir at the extremity of the digestive 

 cavity has changed its outline from a circular to a lobed one (PI. II. Fig. 8, o) ; the 

 lobes widen towards the sides, almost forming diverticula (•«/, w), from the digestive 

 cavity. During this time, the main digestive cavity has entirely lost its cylindrical 

 form ; it has become narrowed at the extremities and bulging in the centre (PL II. 

 Fig. 8, and isolated, Fig. 9). When seen in profile, and comparing it with earlier 

 stages (PL II. Figs. 2, 4, 5, 7, isolated, Fig. 10, a), it is at once noticed that the opening 

 at one end, the present mouth of the larva, has little by little changed from a position 

 at one extremity of the embryo (PL I. Figs. 27, 28, a), to a slightly eccentric one (PL 

 II. Figs. 4, 5, 7). While the present mouth is changing its position from a terminal 

 to an eccentric one, and while the digestive cavity has been expanding at the bottom 

 into a large reservoir, its closed end is bending more and more towards one side 

 (PL II. Figs. 2, 4), until it finally touches the outer wall of the embryo at m 

 (PL II. Fig. 5). At this point of junction an opening is formed, leading into the 

 bottom of the digestive cavity (PL II. Fig. 7) ; this second opening (rri) is now 

 the true mouth, and performs hereafter all the functions of a mouth, while the firs1> 

 formed opening of the young embryo (a, PL II. Figs. 2, 4, 5, 7) is restricted in its 

 functions, and performs hereafter only those of an anus ; although in the early 

 stages (PL I. Figs. 25, 26, 27, 28 ; PL II. Figs. 2, 4, 5, 6) it had performed the 

 functions of a mouth. We have thus an apparent anomaly in the fact that the 

 first opening becomes the anus, while the true mouth is only formed afterwards ; 

 but this difficulty is readily explained if we compare the functions of this first- 

 formed opening, the so-called mouth, with what we find among Polyps, where one and 

 the same opening performs the double functions of mouth and anus throughout life. 



The diverticula {w, w', PL II. Fig. 7, 10) do not extend, as would seem when seen 



